All publications disclosed herein, below are incorporated by reference, as if fully set forth herein.
There are many different forms of mass data storage technology used in modern computing. One of the prevailing forms of data recording is magnetic data recording due to its large storage capacity and re-usable recording media. Magnetic data recording may be implemented utilizing different types of magnetic recording media, including tapes, hard disks, floppy disks, etc. Over the years, significant developments have been made to increase the areal data recording density in magnetic data recording to raise its capacity.
One method for increasing the areal density of the magnetic media, especially for hard disc storage, is to use perpendicular recording media, which have been found to be superior to conventional longitudinal media in achieving very high bit density. In perpendicular recording, the direction the magnetic flux entering the media is substantially normal to the recording surface rather than along the recording surface as in conventional longitudinal recording.
A major factor that limits improvement in areal density of magnetic recording is superparamagnetism. Superparamagnetism comes from thermal excitations perturbing the magnetization of ferromagnetic particles (grains) and rendering the magnetization unstable. As the ferromagnetic grain size is reduced for high areal density recording, superparamagnetic instabilities become more of an issue. The superparamagnetic effect is most evident when the grain volume V is sufficiently small such that the inequality KuV/kBT≧60 (thermally stable for 10 years) can no longer be maintained. Ku is the magnetic crystalline anisotropy energy density of the material, kB is Boltzmann's constant, and T is the absolute temperature in Kelvin. When this inequality is not satisfied, thermal energy demagnetizes the individual grains and the stored data bits will be unstable. Therefore, as the grain size is decreased in order to increase the areal density, a threshold is reached for a given material Ku and temperature T such that stable data storage is no longer feasible.
It is conceivable to use a recording material with high magnetic crystalline anisotropy Ku to overcome the superparamagnetic effect. However, an increase in the anisotropy Ku will also increase the switching field, H0, which is required to reverse the magnetization direction and is about twice as large as the coercivity HC of the material. Obviously, H0 cannot exceed the write field capability of the recording head, which currently is limited to about 15 kOe for perpendicular recording.
One method that allows the use of high Ku recording material without an equivalent increase in the switching field H0 is to tilt the easy axis of the recording material with respect to the surface normal of the recording medium surface. It is found that a 45°-tilted magnetization angle with respect to the medium surface normal may result in a required switching field H0 that is approximately half the anisotropy field of the media. In other words the switching field H0 for a recording material with a 45° tilted magnetization angle is about half the switching field for the case of 0° tilt angle. In addition, if aligned in the cross-track direction, a tilted magnetization angle may also help increase the signal-to-noise ratio of the recording media and recording narrow tracks by reducing track edge writing. See, “Transition Jitter Estimates in Tilted and Conventional Perpendicular Recording Media at 1 Tb/in2”, Kai-Zhong Gao, Neal Bertram, IEEE Transactions on Magnetics, 39, 704-709 (2003); and “Track Edge Effects in Tilted and Conventional Perpendicular Recording”, Kai-Zhong Gao, Xiaobin Wang, Neal Bertram, Journal of Applied Physics, 93, 7840-7842 (2003).
It has been proposed that thin-film deposition at an angle to the substrate surface (oblique incidence) to create an interlayer with a tilted preferred orientation would facilitate the growth of the tilted magnetic layer. A magnetic layer that is subsequently grown on top of the interlayer may take on the tilted orientation wherever hetero-epitaxy is available. However, the oblique incidence approach may cause a large angular dispersion around the tilted preferred orientation. It is conceivable that a collimator may be used to reduce the easy-axis angular spread around the preferred orientation. However, using a collimator will significantly reduce the deposition efficiency of the magnetic material.
Another drawback with using oblique incidence deposition over a substrate, and in particular a circular substrate with circumferential recording tracks, is that the tilted magnetization direction will be largely along the down-track direction in one quadrant of the substrate and substantially along the cross-track direction in an adjacent quadrant. Having a tilted magnetization angle that is along the down-track direction will cause demagnetization field at transition regions, which limits the linear density. On the other hand, having a tilted magnetization angle that is along the cross-track direction will cause non-symmetric track profiles.
Rotating the substrate during deposition may solve the non-uniformity problem, making the tilted magnetization angle along either down-track or cross-track direction. However, the other drawbacks mentioned above still exist.
It has been proposed that by using certain bi-crystal structure, symmetric track profile and sharp transitions may be achieved. See, “Bi-crystal Structure of Tilted Perpendicular Media for Ultra-high-density Recording”, Guan, Lijie, Zhu, Jian-Gang, Journal of Applied Physics, 93, (7735-7737) 2003. However, the bi-crystal structure is difficult to implement.
Accordingly, it would be desirable to develop a magnetic recording media that can take advantage of tilted magnetization without the drawbacks of the prior art, such as demagnetization field at transition regions and non-symmetric track profiles.